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What is TypeScript?

Learn that TypeScript is a typed superset of JavaScript.

What is TypeScript?

TypeScript is a typed superset of JavaScript. That is, every valid JavaScript program is also a valid TypeScript program. TypeScript source files use the .ts extension.

let isim: string = "Ayşe";
let yas: number = 30;

Type annotations are checked only at compile time; the output is still plain JavaScript.

Type annotation syntax

In TypeScript you specify a variable's type after its name with a colon. If you try to assign a wrong type, the compiler warns you.

let puan: number = 95;
puan = "yüksek"; // Error: 'string' is not assignable to 'number'

Type annotations are optional; in most cases TypeScript infers the type from the assigned value.

Practice Your first type annotation

Declare a variable named yas. Its type is number, its value is 25. Then print the yas variable with console.log.

Starter code

index.ts
// yas değişkenini number türüyle tanımla
// değer 25 olsun

// console.log ile yazdır

Sample solution

index.ts
let yas: number = 25;
console.log(yas);

Expected output

25

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What does it mean that TypeScript is a 'superset' of JavaScript?

  • Every valid JavaScript program is also valid TypeScript. Correct answer
  • Every valid TypeScript program is also valid JavaScript.
  • TypeScript completely replaces JavaScript.
  • There is no relationship between TypeScript and JavaScript.

Explanation: A superset includes all of JavaScript's syntax and adds types on top. So every JS code is valid in TS; however TS code with type annotations does not run directly as JS.

2 TypeScript file extension

Question 
// Kaynak dosya: selam.____
let mesaj: string = "Merhaba";
Answer 
// Kaynak dosya: selam.ts
let mesaj: string = "Merhaba";

Explanation: TypeScript source files use the .ts extension. .js is the compiled JavaScript output.

3 TypeScript core concepts

Term Meaning
: string Type annotation
.ts Source file extension
üst küme JS code is valid in TS
JavaScript TypeScript's foundation

All lessons in Beginner

1 TypeScript intro and setup

  1. What is TypeScript?
  2. Why TypeScript?
  3. tsc compiler
  4. tsconfig basics
  5. Summary - TypeScript basics

2 Primitive types

  1. string type
  2. number type
  3. boolean type
  4. let and const
  5. Summary - Primitive types

3 Arrays

  1. Array type
  2. Array methods
  3. readonly arrays
  4. Multidimensional arrays
  5. Summary - Arrays

4 Tuples

  1. What is a tuple?
  2. Tuple usage
  3. readonly tuple
  4. Summary - Tuple

5 Enums

  1. Numeric enum
  2. String enum
  3. const enum
  4. Enum members
  5. Summary - Enum

6 any, unknown, never

  1. any type
  2. unknown type
  3. never type
  4. void type
  5. Summary - Special types

7 Type annotation

  1. Variable annotation
  2. Parameter annotation
  3. Return type annotation
  4. Summary - Type annotation

8 Type inference

  1. Basic inference
  2. Inference with const
  3. Best common type
  4. Summary - Type inference

9 Objects

  1. Object type
  2. Optional properties
  3. Readonly properties
  4. Summary - Objects

10 Functions

  1. Function declaration
  2. Optional and default params
  3. Rest params
  4. Function overloads
  5. Summary - Functions

11 Arrow functions

  1. Arrow syntax
  2. this with arrow
  3. Arrow type
  4. Summary - Arrow functions

12 Interfaces

  1. What is an interface?
  2. Property types
  3. Interface vs type alias
  4. Summary - Interfaces

13 Type alias

  1. What is a type alias?
  2. Composition with alias
  3. Alias vs interface
  4. Summary - Type alias

14 Union and intersection

  1. Union types
  2. Intersection types
  3. Literal types
  4. Summary - Union & intersection

15 null and undefined

  1. null vs undefined
  2. Optional chaining
  3. Nullish coalescing
  4. Summary - null & undefined

16 Strict mode

  1. strict: true
  2. strictNullChecks
  3. strictFunctionTypes
  4. Summary - Strict mode

17 Project: Shopping cart

  1. Cart design
  2. Add product
  3. Total calculation
  4. Apply discount
  5. Integration

18 Project: Todo CLI

  1. Task model
  2. Add and list
  3. Completion
  4. Filtering
  5. Integration

19 Project: Library

  1. Book model
  2. Add and search
  3. Borrow
  4. Return
  5. Integration

20 Beginner final

  1. Challenge 1: Types
  2. Challenge 2: Functions
  3. Challenge 3: Unions
  4. Challenge 4: Mini app